The invention relates to a porous, flow-through molded body, designed specifically for the removal of diesel soot particles from the exhaust gas of diesel engines. It is comprised of an alternately closed honeycombed body made of silicon carbide. The invention further relates to a method for producing this molded body, in which a starting powder of silicon or a mixture of silicon with portions of silicon carbide and/or carbon is combined with an organic binding agent that can be coked and is then molded. It is especially extruded into a green body which is then subjected to a coking treatment in an inert-gas atmosphere; the molded body obtained in this manner is then heated in the presence of nitrogen or all inert gas that contains nitrogen to such a temperature causing the free silicon to be converted with the carbon in a reaction into silicon carbide.
Molded ceramic bodies are gaining increasing importance as filter elements and as supporting material for catalysts, especially in applications involving the filtration of hot gases, because molded bodies of this kind are extraordinarily temperature-resistant. For this reason, their use in the removal of soot particles from the exhaust gas of diesel engines is of particular interest, as these soot particles are believed to produce cancerous effects.
The material silicon carbide has proven to be particularly well-suited for this purpose. It is chemically stable and demonstrates high temperature and thermal cycling resistance. Despite a large number of attempts however, there Continue to be difficulties in producing a molded body from this material that can fulfill all the necessary requirements. Various methods have been applied in these attempts.
In the process specified in DE-C-41 30 630, a starting powder is formed from silicon or from silicon and carbon and/or .alpha.-silicon carbide. This is combined with an organic binding agent that can be coked and a solvent, preferably water. A green body is formed, for example via extrusion, which is then carbonized in an inert-gas atmosphere or in a vacuum by heating it to a temperature between 600.degree. and 1000.degree. C. The molded body obtained in this manner is then subjected to a reaction firing at a temperature between 1400.degree. and 2000.degree. C. The silicon is thus converted with the carbon to .beta.-silicon carbide.
For the application of these bodies with diesel engines it is important for the soot particles that are deposited on the molded body to be cleaned off at regular intervals. This way the flow resistance does not become too great. To achieve this, the soot particles are burned off by sufficiently heating the molded body via an electrical current. Silicon carbide, however, has a relatively high resistance thus, high voltages would be required in order to heat the molded body to a sufficiently high temperature. For that reason, the electrical conductivity of the silicon carbide is manipulated by adding various substances to it, particularly nitrogen. In the above-described method, this is accomplished either by adding a suitable compound to the powder or by implementing the reaction Firing in a nitrogen atmosphere or a nitrogenous atmosphere.
In its practical application, the above-described method has its limits. The pore structure that can be achieved via this method enables a satisfactory flow rate only when combined with extraordinarily narrow wall thicknesses of less than 1 mm. And such narrow wall thicknesses present difficulties even in the production of the green body. Even more serious is the factor that a molded body of this kind does not possess sufficient strength. And a molded body of this type would be exposed to considerable vibration if used in diesel engines.
In order to achieve flow-through properties that will fulfill the necessary requirements with sufficient strength, attempts have been made using a second variation on the above-described method. In this variation, first a granulate is formed. It is then molded into the green body via quasi-isostatic pressing. Disregarding the fact that this requires an additional step in the procedure, this results in wall thicknesses that are in the centimeter range. Although the wall thicknesses can be reduced via a machining process, for reasons of stability they can be reduced by only one-half at most. And even then the volume of ceramic building material per filter surface area is relatively great, particularly since only simple pipe geometries can be produced with this method. Added to this is the fact that the amount of energy required for the regeneration is high when the specific resistance is decreased via endowment with nitrogen. In addition, the supplementary machining of the molded body makes production more costly.
Another attempt is made with the method specified in EP-A-0 336 883. Insofar as silicon carbide molded bodies are recommended in that patent, primary silicon carbide powder, in the particle size range of 75 to 170 .mu.m, is used as the starting material. The powder, combined with a binding agent, is molded into a green body which is then heated to a temperature between 1500.degree. and 1900.degree. C. in order to eliminate or temper the binding agent. This way the silicon carbide particles will be bonded together via an external phase.
With this method, as with the previous method, no satisfactory molded body can be produced. The thermal and chemical resistance are negatively affected by the external phase. A high degree of strength or electrical conductivity cannot be generated. In the sintering, a linear shrinkage of 15% or more usually occurs, with the result that the production of geometries that are dimensionally accurate and without deformation is difficult. In addition, the flow-through properties achieved with the use of relatively coarse SiC particles are unsatisfactory, unless in this case, as before, very narrow wall thicknesses, which cannot fulfill stability requirements, are used. On the other hand, the size of the silicon particles cannot be optionally increased as this will also result in stability problems. A further disadvantage consists in that silicon carbide powder is extraordinarily abrasive. This results in high levels of wear in preparation of the body, in its extrusion, and in any possible processing steps.
In the method specified in WO 93/13303, silicon carbide particles in a bimodal particle-size distribution of approximately 70% by weight oversize material, ranging in size between 35 and 125 .mu.m, and approximately 4 to 13% undersize material, ranging in size between 0.3 and 2 .mu.m are used as the starting material. Following the addition of a binding agent, a green body is molded and heated between 300.degree. and 500.degree. C. in order to burn off the binding agent. The molded body formed in this manner is then heated to a temperature that is greater than 2200.degree. to 2600.degree. C. in order to decompose the undersize material. The decomposition product settles out in an evaporation-condensation mechanism as a sublimate on the contact points of the coarse particles, thus creating stable bridges between these particles.
This method offers the advantage that no shrinkage occurs. Therefore, molded bodies with good flow-through properties and strength can be produced. The disadvantage of this method, however, is that extraordinarily high temperatures, in the range of 2500.degree. C., are required for starting up the evaporation-condensation mechanism. This requires a correspondingly high expenditure of energy. In addition, the starting material generates high levels of wear in the body preparation, the extrusion of the molded body, and any possible machining steps. The undersize material required in every case is relatively expensive. And the bimodal particle distribution results in the danger of a "phase separation" in the production process.